How do they make the first down line appear on the football field during a game?

Random question, but it was bugging me....... How do they make the first down line appear on the football field during a game... or other markers on the pitch, say in Rugby or cricket.
I thought I knew how it was done, but then kept thinking, no it can't be that easy?!!? This short and sweet article answered exactly what I was after ... :)

http://www.scienceline.org/2008/01/14/ask-tweed-yellowline/

http://upload.wikimedia.org/wikipedia/en/c/cb/First-down-line.jpg

Then looked up some other technology that looked rather interesting too... 

Hawk eye
http://en.wikipedia.org/wiki/Hawk-Eye

Neat, huh!